Optical devices and methods of manufacture thereof

ABSTRACT

An optical fiber has a core, an inner cladding and an outer cladding on. In a first region the inner cladding refractive index has a first value which differs from the core refractive index by about 0.0045. The inner cladding refractive index changes along said fiber from the first value to a second value at a free-space end. The second value matches the core refractive index, so that the effective diameter of the core at the free-space end is that of the core plus the inner cladding. Hence the outer cladding acts to constrain the light in the end region. The spot-size and the V number (normalized frequency) are increased, thereby increasing offset tolerances.

This application is a 371 of PCT/GB00/04634 filed on Dec. 4, 2000.

The present invention relates to optical devices and in particular, butnot exclusively, to optical fibres. The present invention also relatesto methods for the manufacture of optical devices.

Reference is made to FIG. 1 which shows the cross section through anoptical fibre. The optical fibre 2 has a core 6. The core 6 issurrounded by an inner cladding 4 which in turn is surrounded by anouter cladding 8. The outer layer of the optical fibre is a sheath 10which protects the optical fibre. The refractive indices of the core 6and the inner cladding 4 are selected along with the core diameter sothat a light beam will travel substantially down the core of the fibre.The size of the light beam as it travels down the core is determined byboth the refractive index difference between the core 6 and the innercladding 4 and the core diameter. The beam size is sometimes referred toas the “spot size”.

Optical fibres are widely used and it is often necessary to join twofibres together. This may be using a splicing technique, a single fibreconnector or a more complicated connector having an array of fibres,which are to be joined to another array of fibres. A good connectionbetween the fibres is required in order to reduce insertion losses. Inpractice, it is difficult to achieve this, particularly where more thanone optical fibre is involved. Optical fibres often provide inputs todevices in which the light from the input fibres are directed to one ormore output fibres via an intermediate arrangement. The intermediatearrangement may comprise one or more components such as a lens or adiffraction grating or a beam steering element. This type of productsuffers from the same problems.

It is therefore an aim of embodiments of the present invention toaddress this problem.

According to one aspect of the present invention, there is provided aoptical device comprising a core portion having a first refractiveindex, said core being arranged to permit a light beam to traveltherethrough; and an outer portion having a plurality of differentrefractive indices along a longitudinal axis of the device, saidrefractive indices being such that the light beam is arranged to travelalong said device.

According to a second aspect of the present invention, there is provideda method of manufacturing an optical device as claimed in any precedingclaim, wherein said outer portion is arranged to be sensitive to apredetermined type of electromagnetic radiation, said method comprisingradiating said outer portion with said predetermined type ofelectromagnetic radiation to thereby provide said plurality of differentrefractive indices in said outer portion.

According to a third aspect of the present invention, there is provideda fibre holder joining a plurality of optical devices, said connectorcomprising a first relatively thin plate and a second relatively thinplate, said first and second plates being separated by a relativelythick layer, said first and second plates having a plurality of holestherethrough for accommodating said optical devices and said thick layeralso having a plurality of holes therethrough for accommodating saidoptical devices, wherein the holes through the first and second platesare smaller than those through the thick layer.

For a better understanding of the present invention and as to how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings in which:

FIG. 1 shows a cross section through a known optical fibre;

FIG. 2 illustrates a first method embodying the present invention;

FIG. 3 shows a second method embodying the present invention;

FIG. 4 shows a cross section through a connector or fibre arrayembodying the present invention;

FIG. 5 shows a micro-electromechanical device embodying the presentinvention;

FIG. 6 shows a graph of coupling efficiency against transverse offsetfor a known optical fibre and fibres embodying the invention;

FIG. 7 shows a graph of coupling efficiency against longitudinal offsetfor a known optical fibre and a fibre embodying the invention;

FIG. 8 shows a graph of coupling efficiency against lens tilt for aknown optical fibre and a fibre embodying the invention;

FIG. 9 shows a wave guide embodying the invention;

FIG. 10 a shows a multiplexer embodying the invention;

FIG. 10 b shows a demultiplexer embodying the invention;

FIG. 11 shows how a fibre pigtail is attached to a wave guide;

FIG. 12 shows how slab and channel wave guides can be connected;

FIG. 13 illustrates a desired shape;

FIG. 14 illustrates photo-sensitivity characteristics for an example;

FIG. 15 shows an example fibre index at the end of a taper;

FIG. 16 illustrates an undesirable shape;

FIG. 17 shows a cross-section through an example fibre;

FIGS. 18 to 20 show graphs of normalised index change against radius;

FIG. 21 illustrates an example of the refractive indices at points inthe core, inner cladding, and outer cladding;

FIG. 22 shows an example beam-steering element array; and

FIG. 23 illustrates an example of a beam steering array using con-focallenses.

Reference will first be made to FIG. 2, which shows a first embodimentof the present invention. A fibre 12 has a core and inner cladding asshown in FIG. 1. The core is silica doped with alumina or any otherrefractive index changing dopant. It should be appreciated that the corecan be of any suitable material. However, in preferred embodiments ofthe present invention, the core is not photosensitive. However, itshould be appreciated that in some alternative embodiments of thepresent invention, the core may be photosensitive.

The inner cladding is again silica which has been doped with aphotosensitive material such as germanium and possibly boron and/orfluoride to alter the net refractive index of the cladding to therequired initial refractive index. It should be appreciated that inembodiments of the present invention, the cladding can be of anysuitable material which is photosensitive, for example, phosphorous orany other material. In preferred embodiments of the present invention,the cladding is photosensitive with respect to ultra violet radiation.However, in alternative embodiments of the present invention, thecladding may be photosensitive to other frequencies or may be sensitiveto other types of radiation. To avoid difficulties, the cladding ispreferably not sensitive to the frequency of the radiation, which is totravel along the core of the optical fibre.

As can be seen from FIG. 2, the fibre has a longitudinal orientationrepresented by axis 14. The end region 11 of the fibre 12 is radiatedwith UV radiation in a direction perpendicular to the longitudinal axis14 of the fibre 12. This is represented by the arrows referenced 16.

The source of the UV radiation can be any suitable source. However, inpreferred embodiments of the present invention, the UV source isprovided by a laser 18. The laser 18 can be of any suitable laser but inpreferred embodiments of the present invention is a copper vapour laser.The copper vapour laser has been found to be preferable to other typesof laser in that it has a high average power but a medium peak power.Frequency doubled copper vapour lasers are able to achieve this as theyhave a relatively high pulse repetition rate. One example of a coppervapour laser is one manufactured by Oxford Lasers. Of course in othersituations, other types of lasers may be appropriate.

In one embodiment of the present invention, the time-averageddistribution of the UV radiation applied is approximately, to the firstorder, Gaussian. In other words, the end of the fibre 12 receives thegreatest amount of UV radiation with the radiation received by the fibre12 decreasing according to the Gaussian distribution with increasingdistance from the end of the fibre. Any other distribution can bealternatively used which preferably provides a levelled out middle parton the beam and a beam tail which tends to zero. For example a cosinetype intensity variation can be used or part of the cosine shape. Thebeam may be in the form of a parabola. The slope tending to zero can belinear, non linear, steep or shallow. Likewise the portion around thehighest intensity can also be linear, non linear, steep or shallow.

By irradiating the fibre with UV light, a change occurs to the molecularstate of some of the molecules in the cladding. This change alters theproperties of the cladding and, in particular, its refractive index.This can be seen from the graph of FIG. 2 which shows the localrefractive index of the core and inner cladding relative to therefractive index of the cladding far from the end of fibre. At the end20 of the fibre 12, as can be seen from the graph, the differencebetween the refractive index of the core and the cladding is very smallwhilst further along the fibre, the difference is relatively large andof the order of 0.0045. As the refractive index difference at the end ofthe fibre is relatively small, the beam spot size is increased. This isbecause the effective diameter of the core is increased to that of thecore plus the inner cladding.

By increasing the beam spot size, a number of advantages can beachieved. In particular, when the fibre 12 is joined to another fibre,the increased beam spot size makes it easier to join the fibres withoutunacceptable insertion losses occurring. This means that the tolerancesare effectively increased making the assembly costs decrease and thereliability of the connection increase.

As is well-known to those skilled in the art, it is advantageous to soakthe glass in some form of high-pressure hydrogen prior to UV exposure,as the presence of hydrogen diffused into the glass matrix is known toincrease by a very significant amount the photosensitivity of the glass,and hence both the rate of change of the refractive index and theultimate achievable change in refractive index. Preferably the form ofhydrogen used should be deuterium, in order to avoid the introduction ofan excess ‘water peak’ leading to increased absorption at telecommswavelengths close to 1500 nm. This is important in order to minimise theexcess insertion loss introduced by using the device in optical systems.

In embodiments of the present invention, the fibre can be irradiated ononly a part of the circumference thereof. However, in alternativeembodiments of the present invention, the fibre may be radiated on allsides. This may be achieved by moving or rotating the laser beam and/orby moving or rotating the fibre. The refractive index of the fibre canbe uniformly changed or non uniformly changed. In the latter case anelliptical distribution may be achieved or any other suitabledistribution.

In preferred embodiments of the present invention, the fibre is a singlemode fibre. However, in alternative embodiments of the presentinvention, the fibre may have more than one mode.

In the preferred embodiment of the invention, the spot size is increasedby four times. The spot size may be increased by less than this or morethan this in alternative embodiments of the present invention.

The effect of the UV radiation is to provide an up taper in therefractive index of the cladding such that the fundamental mode expandsadiabatically as it travels along the taper to form a near field at thefibre end with a significantly increased spot size as compared to anuntreated fibre. In some experimental results, the transverse offsetbetween two standard fibres (with “dispersion zero” centred at 1300 nm)resulting in a 0.2 dB insertion loss must be less than 1.16 micrometers.In contrast, in an embodiment of the present invention where the beamspot is increased by a factor of 4, the transverse offset can be around4.2 micrometers for the same loss. In other words, the two fibres can beout of alignment in the direction perpendicular to the longitudinal axisby 4.2 micrometers in embodiments of the present invention and stillonly incur at 0.2 dB loss.

Reference will now be made to FIG. 6 which shows a graph of couplingefficiency versus a transverse offset in micrometers for the local orfundamental mode at the end of the taper. This assumes that the index ofthe inner cladding has been increased to match that of the core. Resultsare plotted for various different outer radii of the outer cladding. Thecurve for Vouter equals 2 shows the performance for an unmodifiedstandard fibre where Vouter is:$\left. {{Vouter} = {{\frac{2\pi\quad q}{\lambda} \times \sqrt{\left( n_{CO}^{2} \right.}} - n_{CL}^{2}}} \right)$

q=radius inner cladding/outer cladding boundary

n_(CO)=refractive index of the core.

n_(CL)=refractive index of the cladding

λ=the wave length of light.

In the case of the unmodified standard fibre, the error in thetransverse position must be kept below 1.2 micrometers to maintain aninsertion loss penalty of 0.2 dB or less, as discussed above. Inpractice, this may be achievable for single fibre connectors but may bedifficult to achieve for multi-fibre connections arranged in onedimensional arrays and virtually impossible for two dimensional fibrearrays. For the highest value of Vouter equals 14, the equivalent spotsize is around 20 micrometers (as opposed to around 5 micrometers forthe standard fibre) and the tolerance in the transverse positionincreases to 4.2 micrometers for the same insertion loss penalty of 0.2dB. The equivalent unmodified standard fibre at such a transverse offsetwould have an insertion loss of around 2.7 dB which would beunacceptable.

The graph shown in FIG. 6 thus shows different values for V outer. Ascan be seen from the graph, by increasing Vouter, the tolerance in thetransverse position increases.

The greater transverse offset permitted in embodiments of the presentinvention means that the tolerances of the system are much improved overthe prior art. This makes it easier to connect the fibres, requiringless precision.

Reference is now made to FIG. 7 which shows a graph of the couplingefficiency versus the longitudinal offset. The standard fibre shown withthe solid line is an unmodified fibre whilst the dotted line shows theperformance of a fibre embodying the present invention with a Voutersize of 14. The approximate size of the spot is 20 micrometers. For aunmodified fibre having a spot size of around 5.1 micrometers, thelongitudinal alignment error should be kept below 30 micrometers tomaintain an insertion loss below 0.2 dB. For multi-fibre connectors,this alignment error is the length of the air gap between two facingribbon fibres. As can be seen from the graph, for a spot size of 20micrometers the tolerance in the longitudinal position is increased to350 micrometers for the same insertion loss of 0.2 dB. In a multi fibreconnector, the net effect would be to decrease the variability of theconnection loss between successive matings, to decrease the mean lossand also to increase the device lifetime in terms of the number ofconnections and disconnections which can be performed before it wearsout and the connection loss becomes unacceptable.

In optical systems using the known technology, where an intermediatearrangement is provided between input and output fibres, thelongitudinal position of the fibres often has to be adjusted tocompensate for focal length errors. As can be seen from the graph shownin FIG. 7, the embodiments of the present invention may remove the needto monitor and adjust for the longitudinal alignment for such systems.The systems may be less sensitive to chromatic aberrations and/or lesssensitive to thermal changes, uniform or non uniform.

Reference is made to FIG. 8, which shows lens tilt v coupling efficiencyfor a standard catalogue lens with a focal length of 25 mm. Lens tilt isthe amount by which the rotational axis of symmetry of the lens differsfrom the longitudinal axis of a fibre input to a lens. The lens is usedin the intermediate arrangement of the optical systems discussedpreviously. The calculation assumes a reflective 2f system for theintermediate arrangement. It should be appreciated that a spot size of5.5 micrometers represents that of a standard fibre. The spot sizes of11, 16.5 and 22 micrometers are achieved with different embodiments ofthe present invention. As can be seen, the standard fibre has the worstcoupling efficiency, which gets worse as the lens tilt increases. Thebeam having the spot size of 22 micrometers has the best couplingefficiency with a coupling loss of only around 0.1 dB. As can be seen,the fourfold increase in the spot size results in a fourfold decrease inthe beam divergence making the optical beam steering system much lesssensitive to lens aberrations and lens and fibre array tilts. As can beseen from this graph, the fundamental loss penalty disappears and thisis maintained across a wide range of lens tilts thus removing the needto monitor and adjust the lens tilt.

It is known in semiconductor technology that spot size conversion can beachieved by physically tapering the wave guide core using standardfabrication techniques and this is used to improve the alignmenttolerances for coupling between lasers and amplifiers and single modefibres. However, in fibre technology, a taper in the core size resultsin a change in the fibre outer diameter. This is difficult to controlaccurately. This is in contrast with embodiments of the presentinvention, which permit the taper to be achieved by altering therefractive index. It is therefore preferred in embodiments of thepresent invention that the outer dimensions of the fibre are unchanged.This means that the fibres can be incorporated in one or two dimensionalfibre arrays or connectors at the modified end of the fibre. This alsomeans that the fibre is compatible for splicing purposes with normal orstandard fibres at the unmodified end as well as the modified end.

It should be appreciated that in embodiments of the present invention,the equivalent effect to a down taper in the core size may also beachieved. This may require the core to have a central photo sensitiveregion and an outer non photo sensitive region surrounded by a photosensitive inner cladding. The refractive index of the inner cladding inthe unmodified region substantially equals that of the outer layer ofthe cladding. The effect of the irradiation would be to increase therefractive index of the inner cladding substantially to that of theouter core and raise the refractive index of the inner core to abovethat of the outer core.

In the known arrangements which achieve up taper and down taper byaltering the size of the core, the up taper generally is advantageous ascompared to the down taper, particularly in one dimensional and twodimensional fibre arrays used for beam steering. As the core size isdecreased below its normal value to provide a down taper, the evanescentfield spreads out rapidly. This leads to an increased susceptibility tocross talk from beams directed at adjacent fibres. The overall beamshape also becomes very much less like a Gaussian distribution.Accordingly, for the same spot size in the collimated far field, thebeam divergence is much greater. This limits the propagation distanceinside a beam steering system and hence the scalability. The oppositeeffects occur for an up taper in the core size, that is an increase inthe core size. A fibre down taper in the core size is straight forwardto make whilst a fibre up taper is very difficult. In contrast, themethod proposed in embodiments of the invention is able to achieve anequivalent effect to an up taper in the core size simply. As will bediscussed in more detail hereinafter, this is advantageous for beamsteering and multi fibre connector applications and optical deviceshaving the intermediate arrangement discussed previously. Thermaldiffusion of the core dopant has been used to create a refractive indextaper inside the fibre. However, this technique is difficult toimplement and the improvements which can be achieved are small.Typically, the spot size can be increased by a factor of 2 but not muchmore than this.

In contrast, embodiments of the present invention allow the index changeto be easily controlled and to such an extent as to allow the spot sizeto be increased by a relatively large factor. It is envisaged thatembodiments of the present invention could allow the spot size to beincreased by a factor of 5 or more.

A further advantage of embodiments of the present invention, is that bychanging the relative refractive index between the core and the innercladding, the effective core size of the fibre increases thus reducingcross talk in an array of fibres. The array may be a one dimensional ora two dimensional array.

It should be appreciated that the profile of the ultra violet lightapplied to the fibre in FIG. 2 can be altered so as to provide anydesired profile for the refractive index difference. For example, therefractive index difference may increase at the end of the fibre or maybe uniform along a portion of the fibre. This is controlled bycontrolling the intensity of the ultra violet radiation on differentparts of the fibre.

In alternative embodiments of the present invention, the distribution ofphotosensitive particles in the silica may be controlled so that uniformultra violet radiation may be used to generate the required refractiveindex profile. Alternatively, the distribution of the dopants may have alongitudinal and/or radial or transverse variation.

Reference is made to FIG. 3 which shows a second embodiment of thepresent invention. In this embodiment, the fibre 12 has a core 22 and aninner cladding 24. Ultra violet light is incident on the end of thefibre 12 and is parallel to the longitudinal axis 14 of the fibre. Dueto the absorption of the incident light by the germania molecules, theintensity of the ultra violet light is thus greatest at the end of thefibre and will decrease as the ultra violet light propagates down thefibre. With this technique, it is possible to have the smallestrefractive index difference between core and inner cladding at the endof the fibre with an increasing refractive index difference further fromthe end of the fibre.

Reference will now be made to FIG. 4, which shows a preferred embodimentof the present invention for making a one or two dimensional fibre arrayor connector. The structure comprises first and second outer layers 26and 28 respectively. These outer layers are relatively thin andtypically have a thickness of between 100 micrometers to 600micrometers. It should be appreciated that alternative embodiments ofthe present invention may use other thicknesses for the sheets. Theouter sheets 26 and 28 are ceramic sheets. Ceramic material isadvantageous as it has a low thermal expansion and allows a high packingdensity. The two very thin sheets 26 and 28 are drilled together, at thesame time. In particular, holes are drilled in these sheets throughwhich optical fibres will extend. The ceramic sheets are drilledaccurately, for example with the Oxford Lasers' copper vapour laser.Typically, this laser can achieve a centering accuracy of 2 micrometersfor an array of holes drilled across a 25 mm square substrate. Theaccuracy of the hole diameter will decrease with a thickness of theceramic sheets. For example, the accuracy will be between plus or minus0.25 micrometers for a 100 micrometer sheet to between plus or minus 3micrometers for a 600 micrometer sheet.

Between the two layers 26 and 28 is a middle layer 29. The middle layer29 is again of a ceramic material. The ceramic material has holesdrilled there through in order to accommodate the fibres. The middlelayer is typically much thicker than the outer layers and may be forexample of the order of a millimeter. The holes drilled through themiddle layer are typically bigger than those drilled through the outerlayers. The outer layers are therefore provided to control thetransverse position of the fibres whilst the middle layer is provided tosuppress tilting of the fibres. Because the spot size of the beam isincreased due to the index taper, the accuracy of the position and sizeof the holes in the two layers 26 and 28 can be reduced. In particular,it is now possible to provide a two dimensional array with the requiredaccuracy. For example, with conventional fibres, the accuracy providedby the Oxford Lasers' laser would not be high enough to achieve therequired accuracy for a 32 by 32 fibre switch. However, by increasingthe spot size by a factor of 4, in embodiments of the present invention,this is equivalent to a centering error of plus or minus 0.5 micrometersfor a standard fibre. This leaves a further margin fornon-concentricity, outer diameter and other errors. Embodiments of thepresent invention hence make it possible to provide two dimensionalfibre arrays used for switching and connector applications or anyoptical device having an intermediate arrangement.

By using the very thin sheets for the outer layers, it is possible toprovide the transverse accuracy. The middle layer is thus able tosuppress the tilt.

The decrease in sensitivity to the accuracy of alignment of embodimentsof the present invention means that larger arrays can be achieved withembodiments of the present invention as compared to the knowntechniques.

Reference is now made to FIG. 5, which shows a micro electro mechanicalsystem switch which can advantageously embody the present invention.Large optical switches are generally used in telecommunication networksfor restoration and wavelength routing. It has been suggested that for ahigh number of ports, only free space “beam steering” switches canprovide a sufficiently low cross talk so as to suppress homodyne beatnoise. Micro electrical mechanical systems are attractive as they areamenable to mass production and low cost manufacture. However, therehave been some problems in addressing the packaging of the elementsEmbodiments of the present invention are able to address thisdifficulty.

A micro electrical mechanical system comprises a plurality of inputs 34,which are typically optical fibres and a plurality of outputs 36 whichagain are typically optical fibres. The input and/or output fibres 34and 36 embody the present invention. The switch itself comprises anarray of mirrors. The array has mirrors equal to the number of inputs 34multiplied by the number of outputs 36. Typically the number of inputswill equal the number of outputs. The mirrors are movable between twopositions, a flat position, such as mirror 30 and a switching positionsuch as mirror 32. When the mirror 32 is in the switching position, aninput beam is directed to a corresponding output. Embodiments of thepresent invention are able to improve the tolerance to errors in thepositions of the fibres and micro-lenses. Accordingly, embodiments ofthe present invention allow an array of a size of 32 by 32, or greater,to be realistically achieved.

In preferred embodiments of the present invention, both the inputs andthe outputs would be effectively up tapered towards the end of thefibres. Embodiments of the present invention also would allow the beamdivergences to be decreased which allows a better collimation system. Inparticular, a lens will be provided between the output of each inputfibre and the mirrors. A lens will also be provided at the input to eachoutput fibre from the mirrors. A GRIN lens may be used.

Embodiments of the present invention can also be used with otherconfigurations of the micro electric mechanical switch which are twodimensional. In other words, a parallel layer of inputs, outputs, andmirrors would be provided. This is because the arrangement, such asshown in FIG. 4 allows an accurate two dimensional fibre array to beachieved.

It should be appreciated that the increase in the spot size achieved byembodiments of the present invention allows accurate two dimensionalfibre arrays to be achieved using arrangements other than that shown inFIG. 4.

As will be apparent from the discussions hereinbefore, there are anumber of different situations in which embodiments of the presentinvention can be used.

Embodiments of the present invention may be applicable to optical fibresmade with the modified chemical vapour deposition process. However, itshould be appreciated that embodiments of the present invention can beused with fibres made according to any other suitable technique.

Embodiments of the present invention can make use of the saturationeffect. In other words, the UV light incident on the end of the fibrealters the refractive index by a certain amount and after that point, nofurther change in the refractive index is possible, regardless ofwhether or not the fibre continues to be irradiated.

Using this effect, it is possible to achieve the required refractiveindex a given distance from the end of the fibre by irradiating for apredetermined time without changing the refractive index at the end bymore than a given factor. This effect may be particularly useful withthe embodiment shown in FIG. 3 but may also be used with the embodimentshown in FIG. 2.

Embodiments of the present invention have been described in the contextof a fibre. However, embodiments of the present invention can also beused with other types of wave guide, an example of which is shown inFIG. 9. FIG. 9 shows a dielectric slab wave guide having a first layer40, a second, middle layer 42 and a third layer 44. Layer 42 isequivalent to the core of the fibre shown in the previous embodimentswhilst layers 40 and 44 are equivalent to the inner cladding. The layers40 and/or 44 may be irradiated with ultra violet light so as to modifythe refractive index difference along part or all of the length of thewave guide. It should be appreciated that the light beam will travelthrough layer 42.

Embodiments of the present invention can be used with any other type ofwave guide and not just the slab wave guide shown in FIG. 9.

Embodiments of the present invention are thus able to improve thetransverse tolerance. This makes it easier to connect a fibre inaccordance with embodiments of the present invention to other fibres orconnectors. The other fibres may also be fibres in accordance with thepresent invention.

Methods embodying the present invention are particularly advantageouswhen applied to an array of connectors. The transverse offset toleranceis increased with embodiments of the present invention. This makes oneor two dimensional arrays of connectors easier to manufacture.Additionally, the manufacturing costs are reduced as the arrays are lesssensitive to errors.

Embodiments of the present invention can be used to join a fibre pigtail to the end of a silicon wave guide 100. Because of the reducedsensitivities to errors, such as the transverse offset and thelongitudinal offset it is easier to join the wave guide to the fibrepigtail. The wave guide has three “cores” or channels 104 to which thecores 106 of the fibres are joined. For clarity, the cores are shown indotted lines. It should be appreciated that in practice any number ofcores can be provided and not just three. The channels are surrounded bya cladding layer 102.

As discussed previously, embodiments of the present invention allow twodimensional fibre arrays to be achieved more easily in that thetolerance to location errors of the fibres are reduced by embodiments ofthe present invention.

Embodiments of the present invention can be used with wavelengthmultiplexers or demultiplexers. In particular, FIG. 10 a showsschematically a wave length multiplexer 50 which receives a first fibreinput 52 and a second fibre input 54 and provides a single output 56.The fibres are each connected to a lens 51. Two of the lenses S1 directlight from the input fibres to a diffraction grating which directs lightto the lens associated with the output fibre 56. Embodiments of thepresent invention allow a more efficient multiplexer to be achieved ascompared to the prior art. In particular, the reduced sensitivity totransverse offsets means that the insertion losses between either of theinput fibres 52 or 54 and the output fibre 56 is reduced. The channelbandwidth may also be increased. The longitudinal tolerance is alsoimproved.

FIG. 10 b schematically shows a demultiplexer 58 embodying the presentinvention. The demultiplexer has a single fibre input 60 and two outputfibres 62 and 64 respectively. The demultiplexer 58 allows the inputfibre 60 to be connected to either of the output fibres 62 and 64. Thedemultiplexer includes a similar arrangement of lenses and diffractiongrating as shown in FIG. 10 a. As with the multiplexer 50, embodimentsof the present invention allow a more effective demultiplexer to beachieved as compared to the prior art. Again, this is becauseembodiments of the present invention provide a reduced sensitivity totransverse offsets resulting in a greater coupling efficiency ascompared to the prior art and/or a reduced need for accuracy. Again thechannel bandwidth is increased and the longitudinal tolerances improved.

As discussed previously, embodiments of the present invention areparticularly useful with micro electrical mechanical systems and inparticular may allow larger arrays to be achieved more realisticallythan with the current fibres. As mentioned, embodiments of the presentinvention are much less sensitive to lens tilt.

It should be appreciated that embodiments of the present invention alsoreduce the longitudinal tolerances associated with joining together oftwo fibres or the joining together of two wave guides.

Embodiments of the present invention can be used with high packingdensity arrangements. In order to achieve a high packing density, thefibres or wave guides embodying the present invention can have at leastone tight bend in order to fold the fibre or wave guide into a confinedspace. The tighter the bend, the greater the packing density. For bends,a higher refractive index should be provided in the core. This can beachieved with embodiments of the present invention where a higherrefractive index can be obtained in the core at the bends via ultraviolet radiation.

Embodiments of the present invention can be used with wave guide arrays.A higher refractive index in the cladding can be provided before and/orafter bends to counter the reduction in size of the beam introduced by ahigh core refractive index at the bend. This higher index in thecladding can be at the bend or alternatively down stream of the bend.This can be used to suppress coupling effects introduced by the bendwhere the single mode waveguide may act as a multimode waveguide. Thechange in the refractive index may be done in the plane in which thewave guide bends. The core size may be tapered down. Whilst embodimentsof the present invention have been described in the context of opticalfibres having inner and outer cladding layers, embodiments of thepresent invention are also applicable to optical fibres having only onecladding layer. Embodiments of the present invention can additionally oralternatively be applied to the outer cladding layer if provided.

FIG. 12 shows schematically part of a channel wave guide 100 in dottedlines for clarity such as shown in FIG. 11 joined to a slab wave guidesuch as shown in FIG. 9. The channel 104 of the channel wave guide onlyis shown for clarity. The beam width of the channel wave guide measuredin the vertical direction is typically smaller than the correspondingbeam width of the middle layer of the slab wave guide, therefore byincreasing the spot size in the channel wave guide mismatch losses canbe reduced. The vertical beamwidth of the slab guide can be bettermatched to that of the channel wave guide. This is particularlyadvantageous for arrayed wave guide gratings.

The increased transverse tolerance can be useful in beam steeringdevices which have a phase modulation array between input and outputfibres. Embodiments of the invention result in the beam steering systemhaving a greater wave length range. This is again a consequence of theincrease spot size.

In preferred embodiments of the present invention, the concentration ofgermanium or the like is substantially constant along the length of thefibre or the wave guide. The refractive index profile varies with lengthadiabatically and over a length of order millimeters to tens ofmillimeters.

The optimisation of a device constructed in accordance with the presentinvention is given hereinbelow.

The physical phenomena that give rise to an index change also give riseto depletion of the light causing the index change. Hence it is likelythat the outer regions of the fibre cross-section will receive morelight, and hence change more in refractive index, than the inner regionsof the fibre cross-section. The consequences on the fibre performance,and ways of both combating and harnessing such effects, are discussedhereinafter.

This concerns the shape of the fundamental mode at the free-space end ofthe taper. What is required is a shape that is a smooth bump, with tightevanescent tails, as shown in FIG. 13. As can be seen, the profile has asmooth bump 200, and tight beam tails 202 and 204. Tight tails areadvantageous since they lead to low cross-talk due to coupling frombeams targeted at adjacent similar fibres in a fibre array or connector.In general the higher the fibre V-number, the tighter the tail. A smoothbump is advantageous because it means that the angular spectrum of planewaves is narrow, leading to a low divergence, which improves thescalability of the beam-steering device in terms of numbers of ports.Similarly a smooth bump should have an angular spectrum with minimalside-lobes which would otherwise lead to cross-talk and beam propagationlosses. The evanescent tails lie within the cladding region of a normaloptical fibre. In embodiments of the present invention, at thefree-space end of the taper this region corresponds to the outercladding. The bump lies within the core region of a normal opticalfibre. In embodiments of the present invention, at the free-space end ofthe taper, the effective core consists of the original core and thephotosensitive inner cladding.

With reference to FIG. 14, it is assumed that the photosensitivity ofthe fibre takes some non-zero uniform value at all points within theannulus bounded by circles at radii of r=a and r=b respectively. Hence,within this region the absorption of the incident UV light will beuniform. A simplistic model for the index change, assuming rotation ofthe fibre to enable an index distribution independent of angle θ, isthat the index change is exponential with decrease from b, as given inequation (N.1):Δn(r)˜exp(−a(b−r))  (N.1)

Note: this model ignores conservation of energy

The behaviour can be deduced from the scalar wave equation, as given inequation (N.2): $\begin{matrix}{{\frac{1}{R}\frac{\mathbb{d}}{\mathbb{d}R}\left( {R\frac{\mathbb{d}F}{\mathbb{d}R}} \right)} = {{V^{2}\left( {b^{*} - {s(R)}} \right)}{F(R)}}} & \left( {N{.2}} \right)\end{matrix}$where R is normalised radius, =r/b, F(R) represents the electric field,b* is the normalised eigenvalue, V is the normalised frequency, and s(R)is the normalised shape of the refractive index profile. These symbolsare well-known to those versed in the art of waveguide theory, and aredefined in equations (N.3) to (N.5).

Note for R the outer radius is used, in contrast to the normal practicein the art, because at the free-space end of the taper the effectivecore radius has increased to this outer radius $\begin{matrix}{b^{*} = \frac{N_{EFF}^{2} - N_{CL}^{2}}{N_{PK}^{2} - N_{CL}^{2}}} & \left( {N{.3}} \right) \\{V = \frac{2\pi\quad b\sqrt{N_{PK}^{2} - N_{CL}^{2}}}{\lambda}} & \left( {N{.4}} \right) \\{{s(R)} = \frac{{N^{2}(R)} - N_{CL}^{2}}{N_{PK}^{2} - N_{CL}^{2}}} & \left( {N{.5}} \right)\end{matrix}$where N_(EFF) is the effective fibre index, that determines the phasevelocity of the wave at wavelength λ, N(R) describes the refractiveindex as a function of normalised radius R, N_(CL) is the refractiveindex of the cladding (outer cladding in this case), and N_(PK) is thepeak refractive index within the (effective) core region.

Assuming that the peak UV-written index is equal to that in the originalfibre core, a sample normalised refractive index profile based onequation N.1 is shown in FIG. 15. FIG. 15 shows the fibre index at theend of the taper, assuming uniform photosensitivity. While the originalcore 206 is shown to have a uniform refractive index, throughout therest of the effective core region 208 the index increases with radius.Te outer cladding is generally referenced as 210.

To investigate the effect on the shape of the bump, it is possible tointegrate the wave equation directly to obtain equation (N.6):$\begin{matrix}{{{{R\frac{\mathbb{d}F}{\mathbb{d}R}}}_{R = {R\quad 2}} = {{V^{2}{\int_{R = {R\quad 1}}^{R = {R\quad 2}}{\left( {b^{*} - {s(R)}} \right){F(R)}R\quad{\mathbb{d}R}}}} - {R\frac{\mathbb{d}F}{\mathbb{d}R}}}}}_{R = {R\quad 1}} & \left( {N{.6}} \right)\end{matrix}$By considering equation (N.6) in the limits as R1 tends to zero and R2tends to infinity, it is possible to obtain a simple expression for thisnormalised eigenvalue that shows how it depends on an averaged value ofthe normalised profile shape (s(R)), weighted with the product of radius(R) and the field distribution (F(R)) This expression is given inequation (N.7): $\begin{matrix}{b^{*} = \frac{\int_{R = 0}^{\infty}{{{Rs}(R)}{F(R)}\quad{\mathbb{d}R}}}{\int_{R = 0}^{\infty}{{{RF}(R)}\quad{\mathbb{d}R}}}} & \left( {N{.7}} \right)\end{matrix}$

For the fundamental mode, the field, F(R) does not change sign. Thisequation demonstrates that the normalised eigenvalue, b*, lies between 0and 1. Now, consider equation (N.6), and set R1=zero, in which case thesecond term on the right-hand side of this equation is zero. Hence,without loss of generality it can be assumed to be positive. In theoriginal core region s(R) is set to unity, and hence the integrand inequation (N.6) is always negative, since b can never quite reach unity.Therefore in this region the field is always decreasing with radius,which is what is required for a smooth bump.

Now consider what happens in the UV-written region, again by consideringequation (N.6) but setting R1 to a/b, in which case the second term onthe right-hand side of the equation is the value of the product of theradius and field derivative on entering this region. For a smooth bumpit is desired for the field derivative to remain negative. The integral,however, is likely to be positive, since b* is likely to exceed s(R). Inwhich case R.dF/dR can become less negative and even change sign,leading to a field distribution as illustrated in FIG. 16. The fielddistribution has distortion 212 due to a positive index slope. Such afield distribution has significant side-lobes in the angular spectrum,and will be divergent, leading to poor performance.

As is well-known to those skilled in the art, the normalised eigenvalue,b*, increases monotonically with V number. Moreover, the effect on thefield derivative is proportional to the square of the V number, as shownin the right-hand side of equation N.6. Therefore, the larger the Vnumber, the worse these effects become. However a large V number isprecisely what is required at the end of the taper, in order to obtaintight tails on the field distribution as well as expand the beam width.Therefore it is important to identify how to suppress this index slope.

A technique for minimising distortion of the field distribution due tothe index slope is discussed hereinafter. Given UV illumination from oneside, the light reaching any point inside the fibre depends on how farit has travelled through the photosensitive region. For example, andwith reference to FIG. 17, point P1 at co-ordinates (r, θ)=(b, 0)receives the full incident intensity, I_(MAX), while point P2 atco-ordinates (r, θ)=(b, π) receives an intensity ofI_(MAx)exp(−2α(b−a)). Given that in the linear regime, the index changeis proportional to the incident intensity, this will result in arefractive index profile that varies with angle θ as well as withradius. The effect of rotating the fibre is such that, at any point, thetime-averaged intensity is proportional to the average over θ. This hasthe effect of reducing the UV-written index variation across the fibrecompared to that in equation (N.1).

FIG. 18 shows an example calculation of the index change calculatedusing the time-averaged intensity, for the case of uniformphotosensitivity and therefore uniform UV absorption for all pointswithin the annulus bounded by circles at radii of r=a and r=brespectively. FIG. 18 also shows the index change calculated usingequation N.1. Therefore it can be seen that rotation of the fibre duringUV illumination reduces the refractive index slope and hence improvesthe shape of the field distribution.

Another method to reduce the index slope is by changing the UVwavelength of the beam. In general the UV absorption decreasesmonotonically with wavelength for wavelengths above a peak atapproximately 244 nm. Therefore by choosing a longer wavelength thevalue of a is decreased, and hence the index slope is reduced.

It is also possible to modify the radial photosensitivity profile of thecladding into a series of concentric tubular regions, each with adifferent but substantially uniform photosensitivity, created bysuitable adjustment of the index dopants during their deposition at thepreform stage of manufacture. In the linear regime, the index change ineach region is proportional to the product of the photosensitivity andthe time-averaged intensity, while the UV absorption is proportional tothe photosensitivity. Preferably the photosensitivity should beincreased for each successive tube closer to the optical axis. Hencealthough in each tube the index slope is positive, the aggregated indexslope can be negative or constant, as required for the application. Theresults of a computer model used to test this technique are shown inFIG. 19. The photosensitivity of each region has been adjusted so as toobtain a piecewise continuous approximation to a step-index profile. Aslong as s(R) is always greater than b*, the field derivative will neverchange sign and it should be possible to ensure a smooth bump.

It should be appreciated that other methods for adjusting thephotosensitivity distribution could be used so as to obtain the requiredindex profile after UV irradiation.

It should also be appreciated that the index slope depends on theabsolute as well as relative concentrations of photosensitive dopantswithin the glass.

A further advantage of using a piecewise continuous photosensitivityprofile is that, for a given maximum allowable index slope, it allows ahigher photosensitivity and therefore a reduction in the manufacturingtime. Alternatively, it allows a wider photosensitive region, andtherefore a larger spot size at the free-space end, since the spot sizetends to be of the order of the effective core radius.

Methods to reduce the index slope may also be applied to advantage inthe end-writing method (FIG. 3). In this case what is important is thatthe index should not change too rapidly in the longitudinal direction,so as to suppress coupling from the fundamental mode into higher-ordermodes as it travels along the taper. Preferably the index change shouldoccur over a distance of between a mm or a few tens of mm.

A further method to reduce the sensitivity to the positive index slopeis to change the profile shape so as to reduce the normalised eigenvalueb*. This may be achieved by reducing s(R) such that the index reducessubstantially with radius at the outside of the inner cladding, leadingto an overall shoulder-type profile.

A technique for optimising the amplitude of the UV-written index changerelative to the index of the original core is discussed hereinafter. Ifthe index of the UV-written region exceeds that of the original fibrecore, then there is an effective positive index slope at the originalcore boundary. The normalised eigenvalue is then likely to be greaterthan s(R) at the centre of the fibre. This will cause the fieldderivative to start off positive, leading to gross field distortion.Conversely if the index of the UV-written region is significantly lessthan that of the original core, and the normalised eigenvalue is closeto the normalised index of the inner cladding, there will be a smalltight bump at the centre of the field. Such a tight bump will increasethe width of the angular spectrum. Therefore the index of the modifiedinner cladding should match, as closely as required for the application,the index of the original core. The sensitivity of the fielddistribution shape to the index difference between the UV-written regionand the original core may be reduced to advantage by decreasing thenormalised eigenvalue, by, for example, creating a shoulder-typevariation in index at the outside of the inner cladding. The sensitivitymay also be reduced by decreasing the effective V number at thefree-space end of the taper, as is described further hereinbelow.

A technique for optimising the field distribution for long-distancebeam-steering is described hereinafter A Gaussian field distribution ispreferable for beam-steering applications, due to the low divergence,which improves the scalability of the beam-steering device in terms ofnumbers of ports, and due to the absence of side-lobes which wouldotherwise lead to cross-talk and beam propagation losses. As iswell-known to those skilled in the art, an infinite parabolic indexprofile (with the maximum index at the centre) would give rise to aGaussian fundamental mode. Hence the optimum index distribution at thefree-space end of the taper (after UV writing) is such that the originalcore and inner cladding have a substantially parabolic refractive indexprofile, with the sign of the parabola such that the peak index is inthe core. This could be achieved by continuous or piecewise continuouschanges in the photosensitivity profile of the fibre inner cladding,created by suitable adjustment of the index dopants during theirdeposition at the preform stage of manufacture, or by any other methodof adjusting the photosensitivity distribution. An example is shown inFIG. 20.

A technique for optimising the field distribution for fibre connectorsand short-distance interconnects, which may also be applicable to WDMmultiplexers/demultiplexers, is describer hereinafter. For fibreconnectors the field does not propagate far. For this application it maybe advantageous to use some positive index slope to flatten the fielddistribution around its centre. This makes coupling between such fibresless sensitive to transverse offset. High extinction/low coupling fromadjacent fibres in the connector can be achieved by making the originalfibre have a raised inner cladding, as shown in FIG. 21. Thus theUV-written index profile has a higher refractive index differencebetween effective core and outer cladding, increasing b* and tighteningup the evanescent tails. While at the unmodified end of the fibre,however, the field does not perceive the outer cladding, and so is onlysensitive to the index difference between the core and unmodified innercladding.

There is described hereinafter a technique for optimising the innercladding index dopants so as to improve the final accuracy of themonitoring process, and reduce the drop in index change after annealingand loss penalty. For the index profile at the unmodified end of thefibre to be perfectly compatible with standard fibre, the index dopantsin the fibre inner cladding should preferably be germania to provide thephotosensitivity, together with an index lowering dopant to bring theindex down to match that of the outer cladding. There are two suchcandidates. Firstly boron, which tends to increase the photosensitivityvery significantly, and secondly fluoride, which tends to lower thephotosensitivity. Boron does have the disadvantage, however, ofincreasing the background loss of the fibre sufficiently to preclude theuse of long lengths of such fibre. In practice this means that for manyapplications it would be necessary to have to splice on a length ofstandard fibre to the unmodified end of the index-tapered fibre.Secondly, boron, and especially hydrogenated boron, makes the indexchange decay much more with time and temperature, such that the indexchange that disappears during annealing is more significant, makingcalibration of the manufacturing process more problematic. Flouride doesnot change the background loss of the fibre very much, although it doestend to diffuse. Therefore, despite the lower photosensitivity, the useof fluoride to lower the index down to match the cladding may indeed bepreferable to using boron.

A technique for optimising the final index profile so as to maximise thefinal spot-size is described hereinafter.

Gaussian approximations to the fibre mode are well known in the art.There are many definitions for the spot-size, (ωo, but, independent ofthe definition, the normalised spot size (ω/b for our purposes) dependson the shape of the refractive index profile. Hence, given a maximumvalue of b, the final index profile that maximises the final spot-size,ω, is that which maximises the ratio ω/b. Results in the art indicatethat grading the ‘outside’ of the index profile, leading to a shouldertype profile, decreases the normalised spot size, while a central diptends to increase the normalised spot size. Hence for some applications,what is required is a fairly step-like profile with a weak central dipwhich just flattens the field, but avoids the problem illustrated inFIG. 16.

A technique for optimising the original index distribution of thecladding, so as to maximise the final spot-size, is describedhereinafter. As is well-known in the art, the normalised spot size tendsto decrease with V number. Hence, by adjusting the outer cladding indexso as to reduce the effective V number, it is possible to increase thenormalised spot size and therefore increase the spot-size, given a fixedvalue of b. This may be achieved by raising the index of the outercladding, or conversely, lowering the index of the entire inner claddingand the original core.

In the following discussion hereinbelow, there is discussed thecombination of optical fibres in accordance with the present inventionwith beam-steering optical switches.

Beam-steering optical switches operate in 3-D and therefore allowmassive interconnectivity, with many input and output ports. Typicallysuch a switch consists of one or more fibre arrays, each carryingsignals into or out from the switch fabric, and one or two arrays ofbeam-steering elements. Normally the beam leaving an input fibre isdirected with a suitable optical system towards a beam-steering element,that changes the beam's direction of propagation.

Normally, every input fibre is associated with a particular inputbeam-steering element, and vice-versa. The beam then passes throughanother suitable optical system to a second ‘output’ beam-steeringelement, which then directs the beam through a suitable optical systemto an output fibre. Normally, every output fibre is associated with aparticular output beam-steering element, and vice-versa. Hence, therequired change in direction introduced by the input beam-steeringelement is such that the beam is directed to the output beam-steeringelement associated with the required output fibre. The required changein direction introduced by the second beam-steering element is such thatthe beam is directed into the output fibre with optimum couplingefficiency. In practice there should also be some fine adjustment of theinput beam-steerer so as to maximise the coupling efficiency at theoutput fibre. Suitable elements to implement the beam-steering areminiature adjustable tilt mirrors using MEMS (MicroElectroMechanicalSystems) or Liquid Crystal On Silicon Spatial Light Modulators.

For MEMS systems, typically the optical system used between the fibrearray and beam-steering element array is an array of GRIN lenses, asshown in FIG. 22. FIG. 22 shows a fibre array 220, a GRIN array 222, andan array of beam steering elements 224. The reason for using such asystem is explained further hereinbelow. Such lenses tend to result inpoor overall optical performance: as a single lens element they tend toexhibit significant chromatic aberration, such that there is awavelength-dependent longitudinal offset between the focused output beamand the receiving end of the output fibre. This leads to awavelength-dependent loss that reduces the overall wavelength range ofthe switch for a given optical alignment. Such lenses also tend tointroduce significant spherical aberration, leading to distortion of theoutput spot and limiting the propagation length, and also increasing thespot size back at the output fibre. The limited propagation lengthconstrains the overall size of the switch, and thus limits the numbersof ports that can be supported within a single switch fabric. Suchlenses are also prone to poor manufacturing tolerances. Variationsbetween the individual lenses in an array create variations in thelongitudinal position of the output spot with respect to the outputfibre. This can lead to significant insertion loss variations betweenswitch ports.

As will be described hereinbelow, the use of optical fibres inaccordance with the present invention in such switches allow animprovement in the optical performance, even using GRIN lenses. Inaddition, with the use of optical fibres in accordance with the presentinvention it is possible to enable changes in the optical system thatmake it cheaper, easier to align and better performing.

The use of optical fibres according to the present invention with GRINor other microlenses is now described. An N-fold increase in thespot-size of a beam results in an N-squared factor increase in theRayleigh length of the beam. As is well-known to those skilled in theart, a significant increase in the Rayleigh length of the fibre fielddistribution makes the coupling efficiency into a fibre very much lesssensitive to longitudinal offset. Hence using optical fibres accordingto the present invention with GRIN lenses reduces the sensitivity totheir chromatic aberration and manufacturing tolerances. Furthermore, anN-fold increase in the spot-size leads to a 1/N factor change in thebeam divergence, so the beam should be much less affected by sphericalaberration, allowing greater propagation lengths and lessspot-broadening at the output fibre.

Similar problems (tolerances, spherical and chromatic aberration) applyto other types of microlenses, in which case a combination of opticalfibres according to the present invention and microlens arrays willimprove the optical performance.

As is discussed further hereinafter, optical fibres according to thepresent invention allow use of telescopic systems between fibre arraysand beam-steering arrays, instead of microlens arrays, without undulycompromising the wavelength range (SLMs) and scalability (MEMS, SLMs fora given pixel pitch. Referring to FIG. 23, an alternative optical systemfor directing beams between a fibre array and beam-steering array 234 isto use a pair of confocal lenses 230 and 232 in a telescopicarrangement. Each fibre array is positioned at the input focal plane ofthe lens closest to it, while the output focal plane of the other lensis approximately midway between the arrays of beam-steering elements.The advantages of using such an optical system instead of an array ofmicrolenses is that it avoids the need to align individual microlenseswith each fibre, the lenses can be achromatic and well-corrected forother aberrations, and that the manufacturing tolerances on bulk lensesare much better than for microlenses.

In most cases it is desirable to reduce the required length, L, of theoptical region between the beam-steering arrays, and also to reduce asmuch as possible the physical size of each beam-steering element. For agiven spot-size, ω_(BS) at the beam-steering element, the lower limit tothe element size is constrained by the need to avoid cross-talk due toclipping of the beam tails. Let the width of the beam-steering element,W, be a multiple C of the spot-size, i.e. W=C. ω_(BS). For MEMS switchesused in such a system, for a given number of ports and given maximumdeflection angle introduced by the tiltable mirror, the required mirrorsize is proportional to a function f of the clipping parameter C and ofthe ratio (ω/s), where ω is the spot-size of the fibres in the fibrearray, and s is the separation between the fibres in the fibre array.The particular function is shown in equation N.8: $\begin{matrix}{f = \frac{C}{\left( {\omega/s} \right)\sqrt{1 - \left( {C\quad{\omega/s}} \right)^{2}}}} & \left( {N{.8}} \right)\end{matrix}$

The optimum value of (ω/s) that minimises the function f, and henceminimises both W and L is given by equation N.9: $\begin{matrix}{\left( \frac{\omega}{s} \right)_{OPT} = \frac{1}{C\sqrt{2}}} & \left( {N{.9}} \right)\end{matrix}$

For example, for C=3.4 the optimum value of the ratio ω/s isapproximately 0.2. Typically the fibres in an array will have an outerdiameter of 125 um, or sometimes 80 um, while the fibre spacing insidean array will have slightly larger values. Hence to optimise the elementwidth and interconnect length, the required fibre spot-size is slightlylarger than 27 um, and slightly larger than 16 um, respectively.However, standard telecommunications fibre has a spot-size of onlyapproximately 5 um. Even TEC (thermally expanded core) fibre has aspot-size of only 10 um. Therefore optical fibres according to thepresent invention may be used in such a system to optimise theperformance. Lower values of C can be used, so 3.4 is not the onlypossible value.

Furthermore, in a system using Liquid Crystal Over Silicon Spatial LightModulators, the wavelength range is proportional to the ratio ω/s.Therefore in some circumstances it might be preferable to use values ofthis ratio larger than the optimum for the element width andinterconnect length, in order to increase the wavelength range. Henceoptical fibres according to the present invention may be used toadvantage in such switches in order to obtain a wide wavelength range.

Optical fibres according to the present invention may be used to improveresilience to cross-stalk, due to increased angular selectivity of thefibre. The effective tilt of the output beam-steerer can be used tooptimise the tilt of the output beam directed towards the output fibre.

One side-effect of increasing the fibre spot-size is an increasedsensitivity of the coupling efficiency to the angle of incidence of theincoming beam with respect to the fibre optical axis. In practise thetilt of the beam that is supposed to be steered to the output fibre canbe adjusted to optimise this coupling efficiency. However, in an SLMswitch there are likely to be one or more incoming beams at non-normalincidence, due to unwanted diffraction orders leading to cross-talk. Theincreased angular selectivity of the present invention improves theextinction of these cross-talk beams.

Thus it has been demonstrated that an optical fibre according to thepresent invention has significant advantageous applications. Theapplication of the optical fibres according to the present invention isnot limited to the examples given herein, and may be more widelyapplicable, as one skilled in the art will appreciate.

1. An optical fiber having a core, an inner cladding and an outercladding on and surrounding the inner cladding, the cor having a corerefractive index and being arranged to permit a light beam to traveltherethrough and the inner cladding having an inner cladding refractiveindex, wherein in a first region the inner cladding refractive index hasa first value, and the first value differs from the cor refractive indexby an amount of the order of 0.0045 and in an adjoining region whichincludes a free-space end of said fiber, the inner cladding refractiveindex, changes along said fiber from the first value to a second valueat the free-space end of the fiber, wherein the second valuesubstantially matches the core refractive index, whereby the effectivediameter of the core is that of the core plus the inner cladding at thefree-space end.
 2. The optical fiber of claim 1 wherein the outercladding has a refractive index that is substantially said first valueof refractive index.
 3. The optical fiber of claim 1 wherein the outercladding has a refractive index that is less than said first value ofrefractive index.
 4. The optical fiber of claim 1 wherein the fiber isarranged to have a spot-size at said free-space end that is at leastfour times the spot size in said first region.
 5. The optical fiber ofclaim 1, wherein the outer dimension of the fiber is unchanged along itslength.
 6. The optical fiber of claim 1, wherein fiber is arranged to besingle mode fiber at least in its first region.
 7. An optical fiberhaving a core, an inner cladding and an outer cladding on andsurrounding the inner cladding, the core having a core refractive indexand being arranged to permit a light beam to travel therethrough,wherein the refractive index of the inner cladding varies from a firstvalue substantially equal to that of the outer cladding to a secondvalue at a free-space end of the fiber, the second value substantiallymatching the core refractive index, whereby the effective diameter ofthe core is that of the core plus the inner cladding at the free-spaceend.
 8. The optical fiber of claim 7, wherein the outer dimension of thefiber is unchanged along its length.
 9. The optical fiber of claim 7,wherein the refractive index varies from said first value to said secondvalue by an approximately Gaussian characteristic.
 10. The optical fiberof claim 7, wherein the inner cladding is photosensitive.
 11. Theoptical fiber of claim 7, wherein the inner cladding is photosensitive,said core has a center, and the photosensitivity of the inner claddingreduces with distance from the center of the core.
 12. The optical fiberof claim 7, wherein the inner cladding comprises deuterium loadedgermanosilicate glass.
 13. An optical fiber having a core, aphotosensitive inner cladding and a further cladding surrounding thephotosensitive inner cladding, the core being arranged to permit a lightbeam to travel therethrough, the fiber having a first end zone in whichthe inner cladding has a plurality of refractive indices along alongitudinal axis of the fiber, the fiber further having a second end atwhich it is compatible with a standard telecommunications fiber, saidrefractive indices being such that in use the light beam travels alongthe fiber, the refractive indices defining a taper such that at afree-space end of the fiber the diameter of the core is increased to aenlarged core diameter equal to that of the core plus the Innercladding; and whereby at said free-space end the fiber has a V numbergreater than that of a standard telecommunications fiber, wherein theouter dimension of the fiber is unchanged along its length.
 14. Theoptical fiber of claim 13, wherein the inner cladding has refractiveindices in the said zone which have an approximately Gaussiandistribution between a minimum refractive index and a maximum, saidmaximum being at a free-space end of the fiber.
 15. The optical fiber ofclaim 13, wherein said core has a center and the inner cladding has aphotosensitivity which reduces with distance from said center of saidcore.
 16. The optical fiber of claim 13, wherein the inner claddingcomprises deuterium-loaded germanosilicate glass.
 17. An array ofbeam-steering elements for steering light from an array of input fibersto an array of output elements wherein the input fibers each have acore, an inner cladding and an outer cladding on and surrounding theinner cladding, the core having a core refractive index and beingarranged to permit a light beam to travel therethrough, wherein therefractive index of the inner cladding varies from a first valuesubstantially equal to that of the further cladding to a second value ata free-space end of the fiber, the second value substantially matchingthe core refractive index, whereby the effective diameter of the core isthat of the core plus the inner cladding, wherein each saidbeam-steeling element has a width defined as a given multiple of thespot-size of the input fibers, and wherein the input fibers have a ratioof separation-to-spot-size which ratio is selected to be equal to thegiven multiple times the square root of 2.